Arc cast ingot

ABSTRACT

A VACUUM ARC MELTED INGOT CENTRAL PORTION OF A HIGH TEMPERATURE ALLOY BASED ON ONE OF THE REFRACTORY METAL ELEMENTS OR ON FE, NI OR CO IS SUBSTANTIALLY CRACK-FREE THROUGH PROVISION OF AN INTEGRAL INGOT TOP PORTION OF AT LEAST ONE INCH NOMINAL THICKNESS OF THE UNALLOYED ELEMENT ON WHICH THE ALLOY IS BASED.

Oct. 19, 1971 w. H. CHANG 3,613,210

ARC CAST INGOT Filed June 6, 1966 United States Patent O 3,613,210 ARC CAST INGOT Winston H. Chang, Cincinnati, Ohio, assignor to General Electric Company Filed June 6, 1966, Ser. No. 555,483 Int. Cl. B21e 1/00, 27/00 U.S. Cl. 29-187 3 Claims ABSTRACT OF THE DISCLOSURE A vacuum arc melted ingot central portion of a high temperature lalloy based on one of the refractory metal elements or on Fe, Ni or Cois substantially crack-free through provision of an integral ingot top portion of at least one inch nominal thickness of the unalloyed element on which the alloy is based.

This invention relates to consumable electrode arc melting and casting of metal alloys. More particularly, it relates to an improved arc cast ingot of high strength, elevated temperature resistant alloys such as the refractory alloys and the super alloys and to the method and improved electrode for making such an ingot.

A number of high strength alloys based on the bodycentered-cubic refractory metals such as molybdenum base alloys have been identiiied as useful high temperature structural materials. Such materials, which generally are strengthened through carbide dispersion or solution strengthening or both, can `be used in power producing apparatus such as gas turbines either in a non-oxidizing environment or with added protection from oxidizing conditions. These types of materials are most conveniently consumably arc melted and cast in a vacuum furnace employing an electrode including base and alloying elements which are melted and fall into a mold within the Vacuum arc casting furnace.

Unfortunately, this type of alloy ingot which results after solidificationi in the mold suffers from cracking which is initiated generally at the top but may propagate through a significant length of the ingot.

While the detailed mechanism of cracking is not fully understood, a major contributing factor seems to be the high thermal stresses consequential to the rapid and heterogeneous solidiication characteristics of arc melting. The ability of high strength alloys, and particularly the high strength alloys based on the refractory metals, to accommodate such stresses is limited by matrix rigidity. This is especially true when a fine carbide precipitation occurs upon cooling through the intermediate temperatures.

During arc melting and solidifcation, most of the heat is extracted radially through the sides of the mold. Complicated furnace design and melting procedures to curtail radial heat fiow have often been suggested to overcome the ingot cracking problem. However, the fact that cracking is usually confined to the upper portion of the ingot suggests that sufficient downward (axial) heat ow exists. Apparently this downward flow enhances the crack resistance of the lower portion by reducing the thermal stresses and agglomerating the carbide precipitation.

A principal object of the present invention is to provide a crack-free arc melted ingot including a means for stress equalization and carbide agglomeration `at the top of the ingot.

Another object is to provvide such an ingot having a sufficiently large heat reservoir in the form of a ductile and harmless material at the top of the ingot and melted with the alloy material.

These and other objects and advantages will be more readily understood from the following detailed description, examples and drawing which are typical of but not meant to be limitations on the scope of the present invention.

The drawing is a partially diagrammatic cross-sectional view of the electrode of the present invention in position with respect to a mold for use in a vacuum arc melting furnace.

It has been found that ingot cracking in high strength alloys can be prevented through the use of an improved consumable arc melting electrode having a central portion including base elements and any alloying elements from which the alloy is to be made. Attached to this central portion, which becomes the central portion of the ingot, are top and bottom portions on ends opposite one to the other, sandwiching the central portion between them. The material of the top portion is one which does not have a harmful effect on the central portion whether or not it alloys with the central portion. In addition, such top material is suficiently ductile to prevent crack initiation and propagation into the central portion. Preferably, the material of the top and of the bottom portions are selected from the group of alloying base elements, principally the most predominant base element in a ductile condition. The material of the top portion in the solidified ingot is in an amount sufficient to result in a nominal thickness of at least 1 inch and which prevents crack initiation and propagation.

As used in this specification, the term nominal thickness is defined as the thickness of a cylindrical disk of the diameter of the ingot melted from the prescribed amount of a material.

The ingot of the present invention is an arc cast ingot including a top portion, integral with the ingot, of a material which has suflicient ductility to prevent the initiation fand propagation of cracks into the body or central portion of the ingot and is of a nominal thickness, at least 1 inch, to prevent ingot cracking. The method described in connection with the present mvention for vacuum arc melting a metal alloy to eliminate solidication cracks in an alloy ingot includes the steps of preparing for arc melting into the alloy an arc melting electrode including a top portion of such ductile material and of an amount which upon melting and solidification would result in a nominal thickness for the top portion of the alloy ingot, at least l inch, which will prevent crack initiation and propagation after solidiiication, and then arc melting the alloy body and the top portion to deposit the top portion on the alloy body while both are substantially in the molten condition.

The present invention will be more fully understood from the following typical examples. Although these were confined to molybdenum base alloys, it should be understood that the present invention can be used with other refractory metal alloys. Tungsten base alloys, like molybdenum base alloys, are particularly susceptible to ingot cracking. In addition, the invention can be used with high temperature super alloys, for example, those based on iron, nickel and cobalt, provided they are consolidated by consumable arc melting.

A particular material which was selected for investigation was a molybdenum base alloy consisting essentially of, by weight, about ll.5% Ti, .S0-.75% Zr, about `0.050.l5% C with the balance essentially molybdenum. This carbide precipitation strengthened molybdenum base alloy of the Mo-TZC type is sometimes designated as Mo-3 alloy. The Mo-3 alloy is noted for its excellent combination of high temperature strength and low temperature ductility in the wrought conditions. However, it also exhibits a notorious propensity towards solidilication cracking such that repeated attempts to vacuum arc melt sound 3.5 diameter ingots had not met with success prior to the present invention. The material selected from the top Patented Oct. 19, 1971 and bottom portions of the ingot was unalloyed molybdenum in a ductile condition.

The electrodes for vacuum iarc melting were made by sintering into 2" diameter rods molybdenum with the required amount of carbon but no titanium or zirconium. The MoeC rods included a diameter concentric hole along the length of the rod for the subsequent accommodation of the alloying elements. The top and bottom portions of the electrode were made of 2" diameter sintered unalloyed molybdenum which contained less than 50 parts per million of carbon. The configuration of the electrode is shown in the drawing.

Referring to the drawing, `it is noted that the central portion of the electrode has threaded into it a top portion 12 and a bottom portion 14. Through the center of the central portion 10 is a concentric hole 16 along the axis of the electrode for the accommodation of certain of the alloying elements. Other of the alloying ele-ments can be attached at the periphery of the central portion if desired. Also shown lin the drawing is the water cooled copper mold 18 including a base padding 20 of unalloyed molybdenum covered with unalloyed molybdenum chips 22 to facilitate arc starting.

The following Table I gives the dimensions and weight compositions of the electrodes of Examples 1 through 6.

TABLE I.-ELECTRODE DESCRIPTION and actual thickness are given. The actual thickness differs to the extent that the interface between the top portion and the central or body portion of the ingot, as shown by etched specimens sectioned longitudinally of the ingot, is concave upward. The maximum depth of the top portion, measured at the center, is about 11/2 greater than the nominal depth.

In order to obtain the data of Table III, all 6 ingots of Examples 1 through 6 were sectioned transversely and longitudinally. Then they were polished and etched for macroscopic and microscopic examination.

`Confirming previous experience, both untopped ingots of Examples l and 5 and the insufficiently topped ingot of Example 6 (0.5 nominal top portion) were found to have cracked. On the other hand the ingot of Example 3 having a 3" nominal top portion and of Example 4 havin-g a 2.3" nominal top portion were completely crack free. The ingot of Example 2, which is at the lower limit of the invention, showed no cracks upon cutting provided a stress relief heat treatment such as at 3150 for 2 hours was applied. However, both ingots of Examples 5 and 6 cracked upon solidication and thus prior to the time such heat treatment could be used. Thus the preferred top portion for the 3.5 diameter ingots used in the present invention is at a nominal thickness of at least 1 inch and prefer- Portions Bottom Top Central (2.0 da.) (2.0 dia.) T

1 Len., Wt., Len., Wt., Len., Wt., (1.50%), (0.65%)

in. lbs. in. lbs. in.1 lbs.l 1135.2 lbs 26. 0 29. 6 3 3. 8 0 0 0. 444 0. 192 24. l 23. 0 3 3. 7 3 3. 7 0. 420 0. 182 25. 9 29. 5 2. 6 3. 3 9 10. 8 0. 422 0. 192 26. 3 30. l 3 3. 5 7 8. 4 0. 451 0. 195 25. 0 29. 2 3 3. 7 0 0 (l. 43S 0. 190 25. 9 29. 8 2. 9 3. 6 l. 8 I. 6 0. 447 0. 193

rAlloying element added.

The electrodes of Examples 1 through 6 were vacuum arc melted into 3.5 diameter ingots under the conditions shown in Table II. It should be noted that the ingots of Examples l through 6 were melted under virtually identical conditions.

TABLE IL MELTING DATA [Strring amps= l; ref. vo1ts= 20] The following Table III shows the depth of the top portion as well as the internal cracking characteristics of each of the examples.

TABLE IIL-IN GOT DESCRIPTION Depth of top (in.)

Cracking characteristics EX Nominal Actual 'l Surface Internal 1 None None I-Iairline cracks.. Cracks throughout most ingot length.

5. None None No cracks Do.

6 0.5 1% Hairline cracks.. Cracks both above and below Mo/Mo-3 interface, extending about 2% below surface.

2 1 2% do No cracks.

4. 2. 3 4 No cracks.. Do.

Measured at the maximum depth of concave interface.

Under the heading Depth of Top Portion, it is to be .noted that both nominal thickness, as dened before,

ably at least 2 inches in order to provide an adequate heat reservoir to prevent ingot cracking upon solidication.

The interface region between the central and top portions of the ingots of Examples 2, 3, 4 and 6 were examined by a combination of macroscopic, microscopic, hardness and microprobe analyzer techniques. As was mentioned before, the interface is curvical in shape delineated by poro'sities. Diffusion of the alloying elements Ti, Zr and C into the unalloyed Mo extends to about 0.1", accompanied by alloy depletion of less than 0.15 immediately below the interface. Thus the total depth of adversely affected zone into the alloy or central portion due to porosity and alloy depletion is less than 0.25". The width of the affected zone in the ingots of Examples 2, 3, 4 and 6 does not vary significantly with the depth of the top portion. As is well known in the art, this amount of unrecoverable material is considerably less than the amount of material normally unrecoverable in an untopped ingot because of cracking, piping and the like.

EXAMPLE 7 In addition to the 3.5 diameter ingots of Examples l through 6, a 6" diameter and y6" long ingot of Mo-3 alloy was melted in the same manner as described above, resulting in a nominal thickness of the top portion of the melted ingot of 1.4". The electrode preparation was identical to that described in connection with the ingots of Examples l through 6 except that a larger electrode diameter of 2.5 was used in this Example 7. The co-axial hole in the electrode was y0.5" in diameter. i

As to specific details, the 6" diameter ingot of this Example 7 -was vacuum arc melted under the following conditions: current 6500 amps; melting voltage 36 volts; stirring amperage 0.1 amp; vacuum 5 10-10 torr; melting rate 0.49 in./min. or 3.41 lb./min. After withdrawing from the mold, the ingot was placed in a cold hydrogen furnace.

Then it was slowly heated to 3150 F. and soaked for 2 hours.

Metallographic examination of the 6 ingot showed the absence of cracking as in the previous examples. Thus the present invention was shown to be eective in the larger sized ingots through the use of a nominal thickness of only 1.4 for the top portion rather than proportionately greater thicknesses based on the 3.5 diameter ingots. As in the case of the 3.5 diameter ingots, the interface of the 6" ingots was cur-vical in shape, resulting in a maximum actual depth of 2.5 as compared with the nominal depth of 1.4".

According to the practice of the present invention, the use of a sufficiently ductile material which does not have a harmful effect on the central portion whether or not it alloys with the central portion of an ingot when applied in a suicient amount as a top portion of an ingot, will eliminate ingot cracking upon solidification in those types of alloys which are particularly susceptible to solidification cracking. The invention is readily and economically adaptable to existing apparatus and requires no changes in furnace design. It can be used with all high strength alloys which are consolidated by consumable arc melting.I

Therefore, although the present invention has been described in connection with a particular alloy, it will be understood by those skilled in the art, the variations and modifications of which this invention is capable. It is intended by the appended claims to cover all such variations and modifications.

What is claimed is:

1. A vacuum arc melted ingot having a diameter of at 'least about 3.5, comprising integral top, central and bottom portions,

the central portion, located between and bonded with the top and bottom portions, consisting essentially of a high temperature alloy based on an element selected from the group consisting of the refractory metals and Fe, Ni and C0;

the top portion having a nominal thickness of at least l and consisting essentially of the element on which the high temperature alloy is based, the element being substantially unalloyed and in a ductile condition;

the ingot characterized by the substantial absence of cracks propagating between the top and central portions.

2. The ingot of claim 1 in which the refractory metal element is selected from the group consisting of Mo and W; and the top portion has a nominal thickness of at least 1.4".

3,. The ingot of claim 2 in which the refractory metal is Mo alloyed with Ti, Zr and C.

References Cited UNITED STATES PATENTS 1,767,174 6/1930 Gathmann 29-187 2,541,764 2/1951 Herres 75-84 2,919,189 12/1959 Nosser 75-84 3,269,825 8/1966 Vordahl 75--84 2,654,144 10/ 1953 Dornin 29-187 2,867,895 1/1959 Howell 29-184 2,898,672 `8/ 1959 Howell 29-184 2,940,163 6/ 1960 Davies 29-198 X 3,267,529 8/1966 Gruber 75-10 3,271,828 9/ 1966 Shelton 75-10 3,336,120 -8/1967 Yoda et al. 29-198 3,343,593 9/ 1967 Goton 75-10 HYLAND BIZOT, Primary Examiner 

